The present invention relates to electronic code generating circuits, and more particularly, to an electronic code transmitter for use in railroad signaling systems.
Railroad signaling systems have long been incorporated in high speed railroad territories to transmit data to trains travelling along the tracks. The data can contain both information, such as indications of advance traffic conditions, and commands, such as speed control. When displayed to the train engineer, the data assists the train engineer to govern the train movements in accordance with the track condition ahead of the train.
The invention is especially suitable for use in those railroad signaling systems where the data, in the form of coded pulses, are transmitted along the tracks to the train. The coded pulses of certain types are detected by an onboard detection system which is located on the train locomotive, motor, or cab control car. The coded pulses are then decoded and display the appropriate cab signal to the train engineer. The cab signal is a miniature set of railroad signals which are presented inside the train engineer""s compartment.
Coded pulses, which typically are of an on/off direct current energy type with low frequency, have been used in railroad signaling systems for some time. The coded pulses are characterized by two critical components: code rate and duty cycle.
The number of code rates used and the frequency of each code rate varies from system to system. Some systems may input six different code rates onto the rails. Other systems use up to twelve code rates. Currently, Amtrak employs five code rates of 50, 75, 120, 180 and 270 beats per minute which correspond to the frequencies of 0.83, 1.25, 2, 3, and 4.5 Hz, respectively. Each code rate displays its own unique cab signal, except the 50 code rate which is non vital. If the code rate is out of specification, the coded pulses will not be recognized and will be rejected by the train onboard detection system. This will result in the most restrictive cab signal to be displayed, and will cause train delays.
The same problems will arise if coded pulses are output with an inaccurate duty cycle. Duty cycle is understood as on-time percentage of a pulse. An illustration is depicted in FIG. 4. As shown in the bottom graph of FIG. 4, each pulse has a duration of T which consists of an on-time period t1 and an off-time period t2. The ratio of the on-time period t1 and the duration T in percentage is the duty cycle of the pulse. For coded pulses are recognizable by the train""s onboard detection system, the duty cycle should be 50% or near 50%. That means the on-time t1 is desirably equal or approximate to the off-time t2.
Even if the code rate and duty cycle are correct, the train onboard detection system may sometimes still not be able to detect the coded pulses if the waveform of the pulses is distorted. Though other waveforms are available, the square waveform as presented in the bottom graph of FIG. 4 is recommended for most railroad signaling systems.
To meet such strict requirements relating to the correctness of code rate and duty cycle, appropriate code generators are needed. Presently, Amtrak uses expensive mechanical code generators to drive a number of code following relays which are mostly of the electromechanical type. The code following relays repeat exactly the code rate dictated by the code generators. If the output of the code generator is incorrect, the coded pulses input onto the rails by the code following relays will also be incorrect, and will be rejected by the train onboard detection system. It has been observed that many mechanical code generators suffer with inaccurate output after long in-service years under severe weather conditions. These mechanical code generators have a high incidence of failure as well. The heavy load of a large growing number of code following relays to be driven is another reason that makes mechanical code generators unsuitable for use in the railroad signaling systems.
When the need arises to replace the mechanical code generators, electronic versions thereof have been introduced, but at unacceptably high cost.
An object of the invention is, therefore, to provide an effective and inexpensive electronic code transmitter for use in railroad signaling systems.
Another object of the invention is to provide such an electronic code transmitter which is capable of driving a sufficiently large number of code following relays for a long time under harsh environmental conditions, yet still capable of precisely producing desired coded pulses.
A further object of the invention is to provide such an electronic code transmitter to be a direct replacement for existing mechanical code generators which become obsolete.
Yet another object of the invention is to provide such an electronic code transmitter of universal circuit design which allows for easy regulation and visual indication of the output code rate.
The aforementioned and other features are accomplished, according to an aspect of the present invention, by an electronic code transmitter for driving a number of low impedance code following relays of a railroad signaling system. The electronic code transmitter comprises a timing circuit and a driving circuit coupled to the timing circuit. The timing circuit generates, at a predetermined code rate, square wave pulses with an approximate 50/50 duty cycle; and feeds the square wave pulses into the driving circuit. The driving circuit, upon receiving the square wave pulses, conducts, at the predetermined code rate, a power source to the low impedance code following relays.
In another aspect of the invention, the timing circuit comprises a timer integrated circuit operating in an astable oscillator mode. Preferably, the timer integrated circuit receives an isolated DC power from a dedicated power source.
Yet another aspect of the present invention relates to an electronic code transmitter for driving a number of code following relays of a railroad signaling system. The electronic code transmitter comprises a timing circuit, a driving circuit, and an impedance balancing circuit. The timing circuit generates, at a predetermined code rate, square wave pulses with an approximate 50/50 duty cycle; and feeds the square wave pulses into the driving circuit. The driving circuit, upon receiving the square wave pulses, conducts, at the predetermined code rate, a power source to the code following relays. The Impedance balancing circuit is coupled to an output of the driving circuit to eliminate electrical noise associated with high impedance loads. Thereby, the electronic code transmitter is capable of driving code following relays of any load impedance.
In another aspect of the invention, the timing circuit comprises a timer integrated circuit operating in an astable oscillator mode. Preferably, the timer integrated circuit receives an isolated DC power from a dedicated power source.
In yet another aspect of the invention, the impedance balancing circuit maintains an adequate output load impedance for the electronic code transmitter. Preferably, the impedance balancing circuit comprises a resistor coupled in parallel with the code following relays.
A further aspect of the present invention relates to an electronic code transmitter for driving a number of code following relays of a railroad signaling system. The electronic code transmitter comprises a timer integrated circuit, a frequency regulator circuit, and a controlling relay. The timer integrated circuit generates, at a predetermined code rate, coded pulses with a predetermined duty cycle; and feeds the coded pulses into the controlling relay. The controlling relay, upon receiving the coded pulses, conducts, at the predetermined code rate, a power source to the code following relays. The frequency regulator circuit, which is coupled to the timer integrated circuit, regulates the predetermined code rate by varying a value of at least one of its components, yet maintaining the predetermined duty cycle.
In another aspect of the invention, the timer integrated circuit is a ""555 type timer configured to operate in an astable oscillator mode. Preferably, the timer integrated circuit receives an isolated DC power from a DC-DC converter.
In yet another aspect of the invention, the electronic code transmitter further comprises a resistor, coupled in parallel with the code following relays, to maintain an adequate output load impedance for the electronic code transmitter.
The above and still other further objects, features and advantages of the present invention will become more apparent upon consideration of the following detailed description of several specific embodiments thereof, especially when taken in conjunction with the accompanying drawings.